Did 1918 flu pandemic discriminate by social class?

Evidence for socioeconomic risk factors has been elusive.

If an influenza pandemic were to hit us
tomorrow, who would need the most help? Obvious answers
include children, the elderly, and people who are already ill. We expect
them to be at higher risk than healthy adults—but that isn’t always the case.

What about the poor? Depending on who you ask,
intuitions vary: some people assume influenza does not discriminate by
social class and that everyone is at risk. Others might guess that
conditions that go hand-in-hand with poverty (like poor access to
healthcare or crowded living quarters) create a higher level of risk.
But everyone is guessing, because evidence on this question has been
surprisingly difficult to pin down.

While many studies have analyzed the risks of
flu on a country or county level, the city-level is where we’d really be
able to compare strong gradients in wealth to risks from the pandemic.
That’s what a recent paper in Proceedings of the National Academy of Sciences of the United States of America (PNAS)
does: it compares census data dating from the 1918 flu pandemic in
Chicago to health records from the time. “We had this great data,” says
Madhura Rane, one of the authors of the paper, “and we thought it would
be interesting to see this association on a small spatial scale.” She
and her fellow authors found evidence that poverty made a difference in
that pandemic.

One difficulty the researchers faced was that
the census data did not include Chicagoans’ degree of wealth or poverty.
The closest datapoints were literacy levels, homeownership, and
unemployment, so the authors used these as proxies. Their analysis
showed that all of these factors made a difference: the risk of dying
from influenza increased with higher levels of illiteracy, unemployment,
and population density. Homeownership was associated with a reduction
in risk.

Nick Wilson is a public health researcher at
the University of Otago who has looked for similar associations between
poverty and the 1918 pandemic, but never found any. He found his own
results surprising “given that nearly all infectious diseases show such
gradients,” he told Ars. According to him, the Chicago research is “the
most scientifically rigorous study to date on socioeconomic gradients
and the 1918 pandemic.”

Why have previous studies not found the same
result? “There are limits with data quality from 1918 for some studies,”
according to Wilson. And in some settings, social disparities may have
been smaller. For instance, in New Zealand in 1918, the social class
gradient might not have been that steep, “so that it might need a very
large study with fine-grained data to detect.”

Rane hopes that the evidence might help to
inform policymakers and public health officials. With the swine flu
pandemic in 2009, she says, health departments faced the problem of not
knowing who should be first in line for the vaccine.

Wilson thinks it’s reasonable to conclude that
both future research and public health responses should focus on social
disparities. Vaccines are always in short supply in a crisis, and
priority groups for vaccination are currently health workers, first
responders, people with pre-existing conditions, and high-risk age
groups “with such groups being prioritized ahead of any groups defined
by social class,” he says. But we’d certainly need more evidence before
findings like these work their way into policy.

Right now, all we know about is Chicago in
1918. A clearer picture will emerge as researchers look not only at
other cities, but also other pandemics. Finding the data for this is
partly a matter of luck. “It’s not easy to do because you need to have
the information available to you,” says Rane, “but when it is
[available], I think it’s worth looking.” What drives the difference is
also not clear: the recent PNAS paper can tell us about the correlations, but it can’t pin down what exactly about being poor made people more likely to die.

Still, studying 1918 is one of the best ways
to equip ourselves to cope with future disaster. Influenza epidemics
will keep coming at us, says Wilson, “typically several times a
century.” We need to understand how they worked in the past to prepare
properly for the next one.